The rule for vector addition may be used to express an arbitrary vector as the sum of multiples of three mutually perpendicular unit vectors. When working

with Cartesian coordinates, these unit vectors are denoted by i, j and k. We

will take them to be directed in the positive sense along the three Cartesian

axes Ox , Oy and Oz , respectively, as shown in Fig. 127. Notice that the axes

are arranged so that the z -axis points in the direction in which a right-handed

screw will advance if turned from x to y. This is called a right-handed set of

axes, and we will always work with such a set. The set of vectors i, j, k is called

a right-handed set of unit vectors or, sometimes, an orthogonal triad of unit vectors, where by orthogonal vectors we mean mutually perpendicular vec-